The present invention relates to a system and related means for monitoring well drilling procedures, drilling rate and depth and for analyzing the condition and content of the earthen formations being penetrated by the well.
Conventionally, the length of the well is determined by measuring the length of drill string extending into the well bore and information regarding the formation condition and content is obtained by analyzing the drilling fluid ("mud") and formation bit cuttings which are returned to the surface from the bottom of the well bore. To this end, the cuttings are analyzed and measurements are made of the amount and type of gas contained in the returning mud, the temperature of the returning mud, the returning mud density, resistivity and various other mud parameters. The information obtained from these measurements is useful in determining that an oil or gas bearing formation has been penetrated and in determining the location and the quantity and quality of the petroleum effluents contained within the formation. The information may also be useful in determining preferred conditions, such as drill bit weight, mud weight and rate of bit revolution required for optimum formation of the well bore.
While equipment and techniques for analyzing the mud and the entrained bit cuttings are conventional there are problems in determining at which subsurface location in the well bore a selected sample of returning well fluid was emitted from the drill bit or a selected bit cutting was drilled away from the formation. In solving these problems, it is customary to calculate the volume of drilling fluid required to displace the cuttings and drilling fluid in the well bore from the bottom of the well bore to the well surface. These calculations employ information regarding the volume of the well bore, the volume of the drilling equipment within the bore and the volume of fluid injected into the well through the drill string. This latter figure is a function of the pump chamber volume and the number of pump strokes occurring in the pumping system used to inject the drilling fluids into the well. Thus, the volume of each pump stroke, the rate and number of pump strokes, the volume of the bore hole, the interior volume of the drill string and the volume occupied by the drill pipe and bit in the hole are employed for calculating the specific subsurface location from which a cutting was drilled and at which a sample of returning fluid taken at the well surface was ejected from the drill bit.
Conventionally, the number of pump strokes required to pump the well fluid and entrained cuttings from the bit location to the well surface is calculated and a pump stroke counter is employed to count the pump strokes. An operator monitors the counter to determine when the calculated number is reached. This calculated number, representative of volume, is referred to as the lag. In practice, the pump stroke counter is initially set to zero with the bit at a known depth point of X feet. When 1 foot (or other desired interval) has been drilled, the bit is at a second lower depth point of X+1 feet and the pump stroke counter reading, for example 100, is noted by the operator and manually recorded. After the lag count is reached, the operator manually removes samples of the returning drilling fluid and cuttings appearing at the well surface during the next 100 pump strokes. Accordingly, these cutting samples are cuttings taken from the bore between the points at X and (X+1) feet and the fluid samples are from the fluid ejected from the drill bit during the time the bit traveled between these same two points.
It will be appreciated that the described conventional procedure requires that an operator know the pump stroke count when the bit is at the beginning of the drilling interval where the sample is to be taken, that an accurate calculation be made to determine the lag, that the samples be taken after the lag has expired and before the fluid and cuttings from a lower interval are returned to the surface and that the samples be properly marked with their interval origin. Because of the need for human intervention, this procedure is often difficult to implement and errors and omissions occur frequently.
In conventional well analyzing and monitoring systems, it is also difficult to appreciate the significance of information obtained from the various recording and monitoring apparatuses employed at the well site. The difficulty stems, in part, from the need to correlate the output data obtained from the monitoring equipment with the bit location and to compare the changes occurring between the drilling fluid being injected into the well and that circulated to the surface. Such correlations and comparisons are made even more difficult by the fact that conventional strip charts are driven at a constant rate while the drilling rate is variable. As a result, the quantity of information appearing within a given increment of chart advance changes with changes in the drilling rate. Since the monitored parameters, such as gas content, mud resistivity, mud temperature, mud density and others must be compensated for lag and drilling rate variations before output data for an identified sample may be correlated with the bit depth at the time the sample was ejected from the drill string and also before the input and output data for such sample may be compared with each other, on-site comparisons and analysis usually are time consuming and require the use of experienced personnel.